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It may take several days to figure out how many people died in Wednesday's collapse of the I-35W bridge in Minneapolis  the official toll has hit five, but nobody expects it to stay there. Between 20 and 30 people are still missing, and while some of them may be lying in hospitals, unconscious and unidentified, plenty of cars are still submerged in the Mississippi River. Anyone trapped inside  and there are such people  is no longer alive. So recovery crews are picking their way carefully around the twisted steel and broken concrete that could shift without warning in the muddy current.

Investigators, meanwhile, from the National Transportation Safety Board (NTSB) and from a private firm of forensic engineers hired by the state of Minnesota, have already arrived to begin trying to figure out what happened. NTSB chairman Mark Rosenker told reporters Thursday evening that the agency plans to use a special risk-analysis software program that will allow investigators to study each functional element of the Minneapolis span. According to Rosenker, the program "can take away every element of the bridge in a computer model, until it falls down." The hope is that the virtual facsimile will help illuminate which component ultimately failed.

That might not be easy. Sometimes a bridge collapses for glaringly obvious reasons  being whacked by a barge, for example. That's what knocked down Florida's Sunshine Skyway bridge in 1980, killing 35 people, and the I-40 bridge near Webbers Falls, Oklahoma, in 2002, killing 14, and a causeway in Louisiana in 1964, killing 14.

But in other cases  the Mianus River Bridge in Connecticut (three dead in 1983) or the Silver Bridge, spanning the Ohio River between Ohio and West Virginia (46 dead in 1967)  the cause is far more subtle. The former was triggered by metal fatigue in a single steel pin: when it finally failed, the loss of support transferred excess stress on other parts, which couldn't handle it, failing in turn. The latter was finally traced, again, to a single piece of metal, which had been forged with a tiny, unnoticed crack that weakened further with corrosion.

Corrosion may have played a role here as well: the Minneapolis bridge  what's known as a deck steel truss bridge  was a concrete roadway supported by gridwork of steel. "When you use both concrete and steel like this," says William Miller, an expert on bridge engineering at Temple University in Philadelphia, "there can be chemical reactions going on where these two very different substances meet. This is especially a problem in extreme climates where water can get into the cracks between supports, freeze and expand and cause a huge amount of damage." Beyond that, says Miller, "concrete is a very forgiving material, and so it can stand up to a lot of cracking and wear. Steel on the other hand, cannot." In a place like Minnesota, where road crews dump corrosive ice melter on roadways by the ton in winter, the problem is even worse.

But since civil engineers know all this, how come they didn't spot the weak points? It may well be that the real breaking point was hidden, or simply wasn't obvious under normal inspection. In fact, as everyone knows by now, the bridge was deemed "structurally deficient" starting in 1990. That didn't result in an emergency repair order, but rather an intention to replace the bridge by 2020  not unusual, evidently, since the designation doesn't suggest imminent danger. According to Minnesota Gov. Tim Pawlenty, speaking Thursday afternoon at a press conference, there are no fewer 70,000 to 80,000 bridges in the U.S. in the same category; at least another 80,000 are considered "functionally obsolete," or not up to current design standards, another label that fails to testify to a structure's safety for travel.

It would be so expensive to fix hundreds of thousands of bridges that it's just not going to happen. But these numbers highlight the problem of the nation's infrastructure. No word is likely to make taxpayers' eyes glaze over more quickly. As a result, officials at all levels of government tend to defer maintenance on bridges and roadways; the voters wouldn't stand for the required expenditures, estimated at more than $9 billion a year. They might, however, be willing to pay for more frequent and thorough inspections, which could distinguish the structurally deficient bridges in imminent danger of failure from those that aren't.

In Minnesota, Gov. Pawlenty announced an immediate emergency round of inspections of all of the state's bridges, starting with the three that have the same structure as the crumbled Minneapolis span.

"The country is behind on infrastructure, and improvements need to be made," Pawlenty told reporters. "Anyone who looks at the national picture or the national statistics and says we don't have problems would be naive."